1. Field of the Invention
The present invention relates to a friction stir joining apparatus for softening, stirring, and joining an object to be joined by frictional heat due to rotation, and a method of controlling the friction stir joining apparatus.
2. Description of the Related Art
There are some methods for joining an aluminum body of a car such as aluminum spot welding and rivet joining. However, problems are imposed that the piping and utility of a robot machine are complicated and the operational environment is bad such as large noise and generation of sparks. In view of the above problems, a spot joining apparatus using friction stir joining method is proposed.
The friction stir joining method presses a workpiece, which is an object to be joined, by rotating a rotor having a pin at its distal end at high speed and executes spot joining by frictional stirring.
A spot joining apparatus using friction stir joining method can be applied to an existing spot welding gun which is used for resistance welding. However, in the spot welding using the spot welding gun, an electrode of the spot welding gun is not rotated. Accordingly, the conventional spot welding control method for the spot welding gun cannot be directly applied to the spot joining apparatus using friction stir joining method.
Further, the conventional spot welding gun servo-controls pressure. Namely, the gun detects pressure in real time during the joining operation and feedback-controls it so as to generate a motor current according to the pressure. This control provides no particular trouble in the welding quality as the conventional spot welding control.
On the other hand, the friction stir joining method has a peculiar behavior property due to a phenomenon relating to frictional heat like frictional stirring, and definition and explication thereof are just started by the persons concerned, and a control method for friction stir joining method is not proposed yet.
Accordingly, a method for applying the existing servo control for a conventional spot welding may be considered as control for a friction stir joining method. Namely, the method monitors the rotational speed in real time during the joining operation and feedback-controls it so as to always obtain a most suitable rotational speed. Furthermore, a method for also monitoring the pressure in real time and feedback-controlling it so as to always obtain most suitable pressure may be considered. However, such servo-control for the rotational speed and pressure is extremely complicated and realization thereof is difficult.
The present invention is intended to provide a friction stir joining apparatus and a method for controlling the same for executing frictional stirring spot joining without practical trouble under simple control.